The present invention relates to an apparatus for displaying images comprising a visual display unit producing an image by means of a raster scan, a page memory storing rows of data and means generating dot video line signals from the stored rows of data in synchronism with the display unit scanning lines. The invention may be used in the kind of apparatus in which an image or a text is compiled using entering means with the aid of a visual display unit (VDU) for facilitating modifications, insertions, deletions and combinations of parts.
There is known for this purpose apparatus of the aforesaid type in which the image can shift on the VDU, either the complete image or part only, that is limited to a window. Normally however, this shifting is too rapid, so that while the scrolling or other shifting is taking place, the image or text is incomprehensible.
In order to obviate this drawback, a visual display apparatus has been proposed in which coded alphanumeric data is recorded in the memory line by line. In each line there is moreover recorded an indication of connection with the following line of codes, so that by varying this indication vertical windows of the VDU can be defined. Moreover, in each line there is recorded a value indicating the number of scanning lines by which the characters in the line of the page memory must be shifted in the corresponding visual display strip. By incrementing or decrementing this value, slow scrolling of the image is obtained in the desired window. To this end, two line buffers are provided in which two lines of the memory are copies alternately, in such manner that each line of codes appears in the two buffers once as the first line and once as the second line. This arrangement requires a very complicated control circuitry and does not allow of varying the height of the characters on the VDU or of handling graphic images, in which the page memory stores each scanning line of an image.